Solar cells are photovoltaic devices that convert sunlight directly into electrical power. The most common solar cell material is silicon, which is in the form of single or polycrystalline wafers. However, the cost of electricity generated using silicon-based solar cells is higher than the cost of electricity generated by the more traditional methods. Therefore, since early 1970's there has been an effort to reduce cost of solar cells for terrestrial use. One way of reducing the cost of solar cells is to develop low-cost thin film growth techniques that can deposit solar-cell-quality absorber materials on large area substrates and to fabricate these devices using high-throughput, low-cost methods.
Group IBIIIAVIA compound semiconductors comprising some of the Group IB (Cu, Ag, Au), Group IIIA (B, Al, Ga, In, Tl) and Group VIA (O, S, Se, Te, Po) materials or elements of the periodic table are excellent absorber materials for thin film solar cell structures. Especially, compounds of Cu, In, Ga, Se and S which are generally referred to as CIGS(S), or Cu(In,Ga)(S,Se)2 or CuIn1-xGax (SySe1-y)k, where 0≦x≦1, 0≦y≦1 and k is approximately 2, have already been employed in solar cell structures that yielded conversion efficiencies approaching 20%. Absorbers containing Group IIIA element Al and/or Group VIA element Te also showed promise. Therefore, in summary, compounds containing: i) Cu from Group IB, ii) at least one of In, Ga, and Al from Group IIIA, and iii) at least one of S, Se, and Te from Group VIA, are of great interest for solar cell applications.
The structure of a conventional Group IBIIIAVIA compound photovoltaic cell such as a Cu(In,Ga,Al)(S,Se,Te)2 thin film solar cell is shown in FIG. 1. The device 10 is fabricated on a base 10A which includes a substrate 11 coated with a conductive layer 13. The substrate 11 may be in various forms and shapes, such as a sheet of glass, a sheet of metal (such as aluminum or stainless steel), an insulating foil or web (such as polyimide), or a conductive foil or web (such as stainless steel) or even cylindrical or spherical parts. Various metallic foil substrates (such as Cu, Ti, Mo, Ni, Al) have previously been identified for CIGS(S) solar cell applications (see for example, B. M. Basol et al., “Status of flexible CIS research at ISET”, NASA Document ID:19950014096, accession No: 95N-20512, available from NASA Center for AeroSpace Information, and B. M. Basol et al., “Modules and flexible cells of CuInSe2”, Proceedings of the 23rd Photovoltaic Specialists Conference, 1993, page 426). The absorber film 12, which comprises a material in the family of Cu(In,Ga,Al)(S,Se,Te)2, is grown over the conductive layer 13, which is previously deposited on the substrate 11 and which acts as the electrical contact to the device. Various conductive layers 13 or contact layers comprising contact materials such as Mo, Ta, W, Ti, TiN etc. have been used in the solar cell structure of FIG. 1. If the substrate itself is a properly selected conductive material, it is possible not to use a conductive layer 13, since the substrate 11 may then be used as the ohmic contact to the device. The conductive layer 13 may be a single layer or a stacked layer. For example, Cr/Mo stacked conductive layer is commonly used in the CIGS(S) solar cell structure because Cr improves adhesion of Mo to the substrate. After the absorber film 12 is grown, a transparent layer 14 such as a CdS, ZnO or CdS/ZnO stack is formed on the absorber film. Radiation 15 enters the device through the transparent layer 14. Metallic grids (not shown) may also be deposited over the transparent layer 14 to reduce the effective series resistance of the device. It should be noted that the structure of FIG. 1 may also be inverted if substrate is transparent. In that case light enters the device from the substrate side of the solar cell.
The first technique used to grow Cu(In,Ga)Se2 layers was the co-evaporation approach which involves evaporation of Cu, In, Ga and Se from separate evaporation boats onto a heated substrate, as the deposition rate of each component is carefully monitored and controlled. Another technique for growing Cu(In,Ga)(S,Se)2 type compound thin films for solar cell applications is a two-stage process where at least two of the components (components being Cu, In, Ga, Se and S) of the Cu(In,Ga)(S,Se)2 material are first deposited onto a substrate, and then reacted with S and/or Se in a high temperature annealing process. For example, for CuInSe2 growth, thin films of Cu and In are first deposited on a substrate to form a precursor layer and then this stacked precursor layer is reacted with Se at elevated temperature. If the reaction atmosphere contains sulfur, then a CuIn(S,Se)2 layer can be grown. Addition of Ga in the precursor layer, i.e. use of a Cu/In/Ga stacked film precursor, allows the growth of a Cu(In,Ga)(S,Se)2 absorber. Other prior-art techniques include deposition of Cu—Se/In—Se, Cu—Se/Ga—Se, or Cu—Se/In—Se/Ga—Se stacks and their reaction to form the compound. Mixed precursor stacks comprising compound and elemental films, such as a Cu/In—Se stack or a Cu/In—Se/Ga—Se stack, have also been used, where In—Se and Ga—Se represent selenides of In and Ga, respectively.
Sputtering and evaporation techniques have been used in prior art approaches to deposit the individual layers or films containing the Group IB and Group IIIA components of metallic precursor stacks. In the case of CuInSe2 growth, for example, Cu and In films were sequentially sputter-deposited from Cu and In targets on a substrate and then the stacked precursor layer or film thus obtained was heated in the presence of gas containing Se at elevated temperatures as described in U.S. Pat. No. 4,798,660. More recently U.S. Pat. No. 6,048,442 disclosed a method comprising sputter-depositing a stacked precursor film comprising a Cu—Ga alloy film and an In film to form a Cu—Ga/In stack on a metallic back electrode and then reacting this precursor stack film with one of Se and S to form the compound absorber layer. U.S. Pat. No. 6,092,669 described sputtering-based equipment and method for producing such absorber layers.
The standard processing approach for the preparation of conductive layers or contact layers that make ohmic contact to CIGS(S) type solar cells is sputtering and the most commonly used contact material is Mo. Electro-beam evaporation has also been used for Mo contact deposition. Sputtering is an expensive vacuum technique with limited materials utilization. Planar magnetron processes typically yield 30-40% materials utilization. A recent publication points out that taking into account other losses in the process Mo target utilization in a sputtering technique may be only about 12% (J. Britt, et al., Proceedings of 4th World Conf. on PV Energy Conversion, p. 388). Although cylindrical magnetron sputtering may increase this utilization to a higher range, cost involved in the preparation of a cylindrical target is higher than the cost of a planar target.
As the brief discussion above shows there is a need to develop new contact materials for Group IBIIIAVIA—based solar cell structures and to develop low cost techniques with high materials utilization to manufacture conductive layers or contact layers that act as efficient ohmic contacts to these devices.